1. Field of the Invention
This invention relates to an additive for non-aqueous electrolyte of a high rate lithium electrochemical cell. More particularly, it relates to the use of alkali metal hexafluorophosphates as additives for stabilizing the cell.
2. Prior Art
It has been known for some time that high rate drain cells having anodes of lithium and electrolyte of solvent and solute are not stable, particularly at high temperatures.
The instability problem is three fold. First, there is a short discharge life after storage due to apparent chemical changes in the electrolyte; secondly, there is a pressure build-up because of detrimental side reactions in the electrolyte; and thirdly, there are cell degradation reactions among cell components. Yet these same unstable electrolyte systems for lithium batteries produce extremely high energy due to high conductivity of the solute in the solvent.
In lithium cells, cell degradation reactions can be manifested in various ways: crust formation on the anode, e.g., with SOCl.sub.2 solutions; gas formation and cell rupture, e.g., with lithium hexafluoroarsenate in methylformate, and polymerization of solvent, e.g., in THF solutions. In all three type degradations, the reaction leads to decreased performance, if not complete failure, of the cell. When using non-aqueous solvents in the electrolyte, both gassing and formation of viscous, if not solid, reaction products near the anode are of major concern; stable or stabilized electrolyte has to be used in lithium cells.
There are many applications, e.g., watch, camera, etc., for which batteries must be constructed in a shape requiring high conductivity to permit efficient and economical utilization of the chemicals. Design limitations require utilization of highly conductive electrolytes. However, conditions of use require electrolytes capable of long term storage.
In U.S. Pat. No. 3,887,397, issued June 3, 1975, to Robert J. Horning, it is disclosed that in lithium cells having a highly conductive methylformate/lithium hexafluoroarsenate electrolyte, the addition of small amounts of lithium tetrafluoroborate will significantly improve the stability of the cell. Thus by using lithium tetrafluoroborate as a stabilizer, Horning succeeded in practically eliminating the gassing reaction between lithium and the specific electrolyte.
However, no mention is made in the patent of any other additives which could be utilized or of any ability to control non-gassing reactions causing deposition of reddish-brown viscous products on and near the lithium. Such viscous deposits will hinder the performance of the anode after prolonged storage and/or at higher rates by slowing ionic mobility near the anode.
It is the objective of this invention to provide a method of stabilizing non-aqueous lithium batteries in terms of reducing chemical interactions between the electrolyte and cell components.
A more particular object of this invention is to provide stabilizing additives for high rate lithium cells so that gassing and other reaction product formations are not observed.